Processes for producing n- and p-conductive layers in semiconductor crystals are described, for instance in "Mikromechanik" [Micromechanics] by Anton Heuberger, Springer-Verlag 1989, chapter 3.1.2, pp. 91-97, in which dopant atoms are purposefully incorporated at predetermined points of the crystal wafer. The doping can be done for instance either by diffusion in from the gas phase or from doped surface linings, or by injecting dopant atoms in the form of externally accelerated ions. Chapter 3.1.5, pp. 114-121, describes various technological processes for depositing monocrystalline silicon layers. In chapter 3.2.1.3, pp. 147-150, the etching process of silicon dioxide (SiO.sub.2), and silicon nitride (Si.sub.3 N.sub.4) in etchants that are anisotropic to silicon is described. Layers of this kind are typically used to passivate silicon surfaces. From chapter 3.2.1.4, page 151, it is known to use pn junctions, to which a voltage is applied, as an etch stop for anisotropic etchants, such as KOH and ethylenediamine solutions.